1. Field of the Invention
This invention relates to an image forming apparatus for forming an image by the electrophotographic process or the like, and a process cartridge detachably mountable with respect to the main body of the image forming apparatus.
Here, the term xe2x80x9cimage forming apparatusxe2x80x9d covers, for example, an electrophotographic copier, an electrophotographic printer (such as an LED printer or a laser beam printer), an electrophotographic facsimile apparatus, an electrophotographic word processor, etc.
Also, the process cartridge refers to at least one of charging means, developing means and cleaning means, and an electrophotographic photosensitive member which is an image bearing member integrally made into a cartridge, which is made detachably mountable with respect to the main body of the image forming apparatus.
2. Related Background Art
An electrophotographic image forming apparatus such as an electrophotographic copier or a laser beam printer, as is well known, uniformly charges an electrophotographic photosensitive member as an image bearing member by the use of charging means, and thereafter applies light corresponding to image information to the surface thereof to thereby form an electrostatic latent image thereon, and supplies a developer to this latent image by the use of developing means to thereby visualize it, and transfers this visualized image to a recording medium, and thereafter permanently fixes it on the recording medium by a fixing device. The recording medium on which the image has been thus fixed is discharged out of the apparatus, and the electrophotographic photosensitive member after the transfer is cleaned by cleaning means.
In such an image forming apparatus, for the purpose of achieving the simplicity of the interchange and maintenance of expendables such as the electrophotographic photosensitive member and the developer, there is a process cartridge system for integrating the electrophotographic photosensitive member, developing means, charging means and cleaning means as process means for acting on the electrophotographic photosensitive member, and further a developer containing container, a waste developer container for containing therein the developer collected from the electrophotographic photosensitive member, etc., as a process cartridge, and making the process cartridge detachably mountable with respect to the main body of the image forming apparatus.
According to this process cartridge system, the maintenance of the apparatus can be done by a user himself without resorting to a serviceman, and the operability of the apparatus can be markedly improved and therefore, this system is widely used in electrophotographic image forming apparatuses.
As described above, the image forming process in the electrophotographic image forming apparatus includes, for example, the step of uniformly charging the surface of an electrophotographic photosensitive member (photosensitive drum) made into a cylindrical shape as a rotary member to predetermined potential.
As one of charging means, there is a method of bringing a roller-shaped charging member (hereinafter referred to as the xe2x80x9ccharging rollerxe2x80x9d) as a rotary member into contact with the surface of the photosensitive drum, and applying to this charging roller a voltage comprising an alternating current bias superimposed upon a direct current bias. Also, in order to obtain the stable charging of the surface of the photosensitive drum at that time, it has been empirically found that it is good to make the discharge current amount between the charging roller and the photosensitive drum equal to or greater than a predetermined value.
FIG. 17 of the accompanying drawings shows the waveforms of the voltage and current of an alternating current bias applied to the charging roller. When an output voltage as shown in FIG. 17A, i.e., an alternating current bias (Vo), is applied to the charging roller, as shown in FIG. 17B, an electric current of the same phase as this alternating current bias (Vo), i.e., a resistive load current (Izr) flowing to the resistive load between the charging roller and the photosensitive drum, an electric current advanced by 90xc2x0 in phase than the alternating current bias (Vo), i.e., a capacity load current (Izc) flowing to the capacity load between the charging roller and the photosensitive drum, and an electric current flowing in a pulse-like fashion during the voltage amplitude peak of the alternating current voltage (Vo), i.e., the discharge current (Is) between the charging roller and the photosensitive drum, flow. Generalizing these, an electric current (Io) of a waveform shown in FIG. 17C flows (total output current). When an alternating current drawn from the charging roller into a high voltage power supply is detected, there is obtained a detected current waveform (Im) of a waveform as shown in FIG. 17D.
FIG. 18 shows the relation between the output current and the amplitude of an alternating current bias (output voltage) applied to the charging roller. It is seen that as the amplitude of the output voltage is gradually increased, the amplitude of the voltage and the output current are substantially proportional to each other for a predetermined voltage amplitude. This is because the resistive load current (Izr) and the capacity load current (Izc) are proportional to the amplitude of the voltage and the amplitude of the voltage is small and therefore, no discharging phenomenon occurs and the discharge current (Is) does not flow. As the amplitude of the output voltage is further increased, a discharging phenomenon starts at a predetermined voltage amplitude (Vs), and the total output current (Io) also deviates from the proportional relation, and thus flows greatly corresponding to the discharge current (Is).
With a view to make the amount of discharge current between the charging roller and the photosensitive drum equal to or greater than a predetermined amount, and stably charge the surface of the photosensitive drum, there has heretofore been a method of providing current detecting means for detecting an electric current outputted from a high voltage power supply for applying an alternating current voltage to the charging roller at a fixed phase synchronized with the alternating voltage applied to the charging roller, and controlling the alternating current voltage the high voltage power supply applies to the charging means so that the detected current by the current detecting means may assume a predetermined control standard value. That is, the alternating current bias applied to the charging roller is controlled so that the value of an instantaneous carrying current (It) which is the addition value of the discharge current (Is) and the resistive load current (Izr), which is detected at the peak of the alternating current bias (output voltage) applied to the charging roller, may become constant.
More specifically, as shown, for example, in FIG. 19 of the accompanying drawings, a high voltage power supply 40 as voltage applying means for applying a voltage to the charging roller has a high voltage transformer drive circuit 41 and a high voltage transformer 42, and a high voltage direct current generation circuit 43 is connected thereto. Further, current detection means 44, a phase detection circuit 45 and a comparing circuit 46 which constitute current detection means 47 are connected to the high voltage power supply 40. The high voltage power supply 40 receives a clock pulse from a printer control portion (not shown) by the high voltage transformer drive circuit 41 and makes a sine wave. Then, an alternating current component is boosted by the high voltage transformer 42, and this high voltage alternating current and a high voltage direct current generated by the high voltage direct current generation circuit 43 are both applied to the charging roller 20 to thereby charge the surface of the photosensitive drum 1. At this time, an electric current detected by the current detection means 44 and phase information detected by the phase detection circuit 45 are inputted to the comparing circuit 46 to thereby detect the instantaneous carrying current (It) during the positive peak voltage or the negative peak voltage of the alternating current voltage. An alternating current bias applied to the charging roller 20 is controlled so that the value of the detected instantaneous carrying current (It) may become constant.
The above-described controlling method according to the prior art, however, has suffered from the following problem.
For example, in an electrophotographic image forming apparatus of the process cartridge type, a process cartridge is repeatedly used for image formation from the initial state in which the image formation has been started, whereby from the initial stage of use until a predetermined developed sheet number is reached, the stains formed by a toner, which is a developer, paper powder, etc., adhere to a charging roller. Also, the film thickness of the surface of the photosensitive drum is decreased with an increase in the developed sheet number.
The resistive load current (Izr) of the charging roller and the photosensitive drum is varied chiefly by these two factors. In this case, images are repeatedly formed, whereby as shown in FIG. 5 of the accompanying drawings, the discharge current contributing to charging is fluctuated by the stain of the charging roller and the decrease in the film thickness of the photosensitive drum. That is, image formation is repeatedly effected from the initial state (state A), whereby the stains formed by a toner, etc., first adhere to the charging roller and resistance rises and therefore, the resistive load current (Izr) becomes small and the discharge current (Is) rises (state B). On the other hand, when the stains on the charging roller are saturated, the scraping of the photosensitive drum decreases the film thickness of the surface thereof and resistance becomes small and therefore, the resistive load current Izr becomes great and the discharge current (Is) decreases (state C).
It is not preferable for the discharge current to fluctuate (increase or decrease) as described above. That is, when the discharge current (Is) between the charging roller and the photosensitive drum increases, the scraping of the photosensitive drum increases and the film thickness of the surface thereof is decreased, thus remarkably shortening the service life of the photosensitive drum. On the other hand, when the discharge current (Is) decreases, defective images such as so-called sandy images are caused by bad charging.
Accordingly, it is an object of the present invention to provide an image forming apparatus and a process cartridge which can suppress any increase or decrease in the discharge current between charging means and an image bearing member by the stains of the charging means and the wear and tear of the image bearing member, and can form stable images for a long period of time.
It is another object of the present invention to provide an image forming apparatus and a process cartridge which can suppress the fluctuation of the amount of discharge current between charging means and an image bearing member due to the stains, environment, etc., of the charging means, and can extend the service life of the image bearing member and obtain a stable quality of image for a long period of time.
It is another object of the present invention to provide an image forming apparatus and a process cartridge which can suppress any increase in the discharge current between charging means and an image bearing member at the initial stage of the use of the charging means and the image bearing member and can suppress any decrease in the discharge current in the vicinity of the service life of the image bearing member.
It is another object of the present invention to provide an image forming apparatus and a process cartridge which can suppress any increase or decrease in the discharge current between charging means and an image bearing member from the turning on of the power supply of the apparatus until a predetermined time elapses.
It is another object of the present invention to provide an image forming apparatus and a process cartridge which can suppress any increase or decrease in the discharge current between charging means and an image bearing member in conformity with each process cartridge mounted with respect to the main body of the apparatus, and can form stable images for a long period of time in conformity with the characteristic of each process cartridge.